1. Field of the Invention
The invention relates to the field of automatic semiconductor bonders, probers, and like devices.
2. Prior Art
In automated semiconductor bonders, wafer probers, and like devices, it is necessary to have a capillary, probe, or similar tool or instrument contact a semiconductor wafer or die. The wafer or die is typically mounted on a work surface such as a bonding table. The height of the die or wafer varies from part-to-part, and thus, the distance the tool or instrument must travel before contact is unkown. For a general discussion of this problem and a proposed solution for an automatic wafer prober, see "Microprocessor Controls Fully Automatic Wafer Prober", Solid State Technology, September 1975 by T. Roland Fredriksen beginning on page 49, particularly see FIG. 7.
For automatic bonders the height sensing problem is more difficult since a bonding tool, such as a capillary, must first contact the semiconductor die and then connect a wire to a lead frame. The height of the lead frame is different than the height of the die. Thus, the Z-drive for the capillary must be capable of sensing two different heights and accurately and rapidly driving the capillary to these different heights. One prior art apparatus for accomplishing this employs two separate cams which drive two arms. Each arm is independently set for a different height.
As will be seen, the presently described apparatus provides unique means for sensing height which additionally provides numerous other features. The disclosed device allows the capillary, by way of example, to make a "soft landing" on the bonding pad since the exact height of the pad is known. Thus a bonding apparatus or other apparatus may operate at a faster speed without striking the wire with an intolerable impact when contacting the bonding pad. Also, after the second bond is made, the capillary may be raised to a controlled distance above this bond before the wire is clamped and then moved an additional distance to break-off the wire. In this manner, a predetermined length of wire projects from the end of the capillary, thereby permitting formation of consistent, molten "balls" needed for the next bond. Without the described height sensing feature this could not be accomplished since the length of wire extending from the end of the capillary is effected by the height of the second bonding surface.